The Institute for Computer Music and Sound Technology has released an open-source Windows framework for developing virtual synthesizers:

SynStart is a simple and straightforward free open source framework for audio programming in C++. It embeds all the interfaces and book keeping routines required to build stand-alone real-time sound generators for the Windows platform: A polyphonic multitimbral voice manager, ASIO and DirectX audio, MME and DirectX MIDI, and a minimalist GUI. It does not provide predefined audio algorithms however. Instead, the developer retains full control over signal processing and can concentrate his design effort on the core of the instrument. There’s an example included that shows how to create a MIDI-controlled polyphonic sine wave synthesizer with 18 code lines. Designed from scratch with focus on efficiency and low latency, Synstart not only makes a practical research tool but an initial point for professional projects as well.